Effect-TS Patterns

Read specific reference files based on your task.

Core Concept: Effect<A, E, R>

Effect<Success, Error, Requirements>
       ↓        ↓           ↓
    "Returns" "Can fail"  "Needs these services"

Generator Syntax (Recommended)

import { Effect } from "effect";

const program = Effect.gen(function* () {
  const config = yield* getConfig();           // yield* unwraps effects
  const db = yield* connectDatabase(config.dbUrl);
  const users = yield* db.query("SELECT * FROM users");
  return users;
});
// Type: Effect<User[], ConfigError | DbError, ConfigService>

Service Pattern Summary

import { Context, Effect, Layer } from "effect";

// 1. Define service with Context.Tag
class HttpClient extends Context.Tag("HttpClient")<
  HttpClient,
  { readonly get: (url: string) => Effect.Effect<Response, HttpError> }
>() {}

// 2. Use in effects (tracked in R position)
const fetchUsers = Effect.gen(function* () {
  const http = yield* HttpClient;
  return yield* http.get("/api/users");
});
// Type: Effect<Response, HttpError, HttpClient>

// 3. Provide layer to satisfy requirements
const HttpClientLive = Layer.succeed(HttpClient, { get: ... });
const program = fetchUsers.pipe(Effect.provide(HttpClientLive));
// Type: Effect<Response, HttpError, never>  ← Requirements satisfied!

Typed Errors

import { Data, Effect } from "effect";

class UserNotFoundError extends Data.TaggedError("UserNotFoundError")<{
  readonly userId: string;
}> {}

const getUser = (id: string) => Effect.gen(function* () {
  const user = yield* findUser(id);
  if (!user) return yield* Effect.fail(new UserNotFoundError({ userId: id }));
  return user;
});

// Catch by tag
getUser("123").pipe(
  Effect.catchTag("UserNotFoundError", (e) => Effect.succeed(guestUser))
);

Quick Reference

When to Read Reference Files

Anti-Patterns (BANNED)

• throw for expected failures → Use Effect.fail(error)
• raw fetch() → Use HttpClient (Guard 44)
• new Date() → Use Clock.currentTimeMillis (Guard 42)
• console.log → Use Effect.log (Guard 26)
• T | null → Use Option<T> (Guard 41)